Jeff,
Your old friend the Universal Carrier did indeed employ an oil bath air cleaner to breath.
Use the same oil as your engine uses, and change at the same interval - or more often if used in dusty conditions.
This, and much more is in the Carrier operators manual. That crusty old curmudgeon
Alex Blair in Ottawa has them as Operating Instructions and Workshop Manual - Universal Carrier Mark I* (Canadian), the "UC-F1".
Stewart